1. Technology Field
The present invention relates to a method of preventing read-disturb errors for rewritable non-volatile memory module and a memory control circuit unit and a memory storage apparatus using the method.
2. Description of Related Art
Digital camera, cell phone, and MP3 player have a rapid growth in recent years and results in the need of storage medium for the consumers. Due to the fact that rewritable non-volatile memory possesses the characteristics of non-volatile data, power saving, small volume, lack of mechanical structure and fast reading and writing speed, rewritable non-volatile memory industry has become a popular role in the electronic industry. For example, the solid-state drive using flash memory as storage medium is widely used as hard drive of host computer to improve access performance of the computer.
NAND flash can be classified into the SLC (Single Level Cell) NAND flash, the MLC (Multi Level Cell) NAND flash and the TLC (Triple Level Cell) NAND flash according to the number of bits stored in each memory cell. Each memory cell of the SLC NAND flash memory stores 1 bit of data (i.e., “1” and “0”), each memory cell of the MLC NAND flash memory stores 2 bits of data and each memory cell of the TLC NAND flash memory stores 3 bits of data.
In NAND flash memory, one physical page is constituted by several memory cells arranged on the same word line. Because each memory cell of the SLC NAND flash stores 1 bit of data, the memory cells arranged on the same word line correspond to one physical page in the SLC NAND flash memory.
Compared with the SLC NAND flash memory, the floating gate storage of each memory cell of the MLC NAND flash memory stores 2 bits of data, in which each storage status (i.e., “11”, “10”, “01” and “00”) includes LSB (Least Significant Bit) and MSB (Most Significant Bit). For example, the value of the first bit from the left in the storage status is LSB, and the value of the second bit from the left in the storage status is MSB. Therefore, the memory cells arranged on the same word line constitute 2 physical pages, in which the physical page constituted by LSBs of such memory cells is referred to as a low physical page, and the physical page constituted by MSBs of such memory cells is referred to as a upper physical page. Particularly, writing speed of the low physical page is faster than that of the upper physical page, and the data stored in the low physical page may be lost when an error occurs in the upper physical page.
Similarly, in TLC NAND flash memory, each memory cell stores 3 bits of data, in which each storage status (i.e., “111”, “110”, “101”, “100”, “011”, “010”, “001” and “000”) includes the first bit from the left (LSB), the second bit from the left (Center Significant Bit, CSB) and the third bit from the left (MSB). Therefore, memory cells arranged on the same word line constitute 3 physical pages, in which the physical page constituted by LSBs of such memory cells is referred to as a low physical page, the physical page constituted by CSBs of such memory cells is referred to as a center physical page, and the physical page constituted by MSBs of such memory cells is referred to as a upper physical page.
However, regardless which kind of memory cell of flash memory module is chosen, after reading the data stored in the same physical block for multiple times, for example, a hundred thousand times to a million times, it is possible that the data read from the physical block is incorrect and the data stored in the block that has been read multiple times may be lost or an error may occur. Such phenomenon is referred to as “read-disturb errors” by one of skilled in the art. Particularly, system information of flash memory storage system, for example, firmware code or file allocation table, may be stored in the flash memory module, and the system information is read in a high frequency during system operation of flash memory. Because of such phenomenon, manufacturers are compelled to develop a mechanism that ensures correct data storage.